1. Field of the Invention
This invention relates to improvements in plasma plating and, more particularly, to a method for forming on a substrate surface thick, binderless, essentially non-porous, fine-grained layers of non-conductive materials. The term "non-conductive", as used herein, refers to materials having a volume resistivity of more than 10.sup.6 ohm-cm. Such materials include semiconductive and photoconductive materials, in addition, of course, to dielectric materials.
2. Description of the Prior Art
In certain electrophotographic and electroradiographic recording processes, it is desirable that the recording element comprise a binderless, thick (i.e. more than 10 micrometers), cohesive, adhesive, non-porous and fine-grained layer of a photoconductive composition. Such a photoconductive layer will exhibit good mechanical integrity, a low dark current conductance, and a high absorption of activating radiation due to its dense structure and thickness. These characteristics are particularly advantageous in electroradiographic applications.
A number of methods are known in the prior art for depositing binderless materials on the surface of a supporting substrate. In one method, commonly known as "vapor deposition", the material to be deposited is placed in a heated crucible arranged within an evacuated chamber and heated to a temperature sufficient to evaporate such material. The substrate is arranged in a position over the crucible and in a spaced relation thereto so that the evaporated material is carried to and deposited on the facing surface of the substrate. While relatively thick layers can be deposited by this technique, there is a tendency, especially with certain photconductive compounds, for the vapor deposited layer to be relatively coarse in structure, exhibiting an open sieve-like pattern with poor adhesion to its substrate and poor cohesion to itself.
In another method, commonly known as "sputtering", a starting material (i.e. the material which is to be deposited) is arranged within an evacuated chamber in spaced relation to the receiving substrate. Typically, the substrate is arranged on a grounded electrode, and the chamber is back-filled with a gas at low pressure. The starting material is positioned on a conductive electrode to which a source of potential is applied. The potential source serves both to produce a gas plasma between the spaced electrodes, and to cause gas ions from the plasma to bombard the surface of the starting material. Such ion bombardment acts to break or knock off atoms or molecules of the starting material, causing them to fly in all directions. Some of these atoms or molecules of the starting material settle on the substrate to form a layer. While the sputtering method has been successfully used for producing relatively dense layers on a substrate, it is basically inefficient and, hence, its use has been limited to the production of layers having a thickness of less than 10 micrometers (.mu.m). Such thin layers are unsuitable for radiographic applications.
In still another method, commonly known as "ion plating", a conductive starting material is placed within an evacuated chamber opposite a conductive receiving substrate. A high voltage DC field is produced between the starting material and the substrate, the latter being the cathode of a high voltage DC circuit, and the chamber is then back-filled with a gas to a pressure sufficient to generate and sustain a plasma discharge. The starting material is then vaporized to form a vapor deposit of the starting material on the substrate. In the presence of the plasma, a portion of the vaporized starting material becomes ionized, and the positively charged evaporant ions and positively charged gas ions are accelerated by the electric field and bombard the substrate surface to densify the vapor deposited coating. While the ion-plating process is useful in forming thick, dense, binderless coatings of conductive materials on conductive substrates, it cannot be used when either the coating material or the substrate is a relatively non-conductive material (e.g. photoconductive, semiconductive or dielectric materials). One finds that if the conductivity of either the coating material or the substrate is such that the positive charge build-up on the deposit (or on the substrate) causes the resulting potential to exceed approximately 40% of the applied potential, the requisite plasma discharge will be extinguished, thereby terminating the coating process. Furthermore, a DC generated plasma tends to produce pinholes or small voids in the deposited layer.
U.S. Pat. No. 3,419,487 to Robbins et al discloses a method of depositing thin film semiconductor coatings on a substrate wherein vaporized raw materials containing the necessary elements to form the desired semiconductor material are introduced into a reaction chamber. While the vaporized raw materials deposit on the substrate, an electric field is produced to attract electrons from a gas plasma or glow discharge toward the substrate surface. As the electrons bombard the substrate, they cause the vapor deposited materials to react on the substrate surface to form the desired semiconductor layer. While the electron bombardment process of Robbins may be effective to produce thin films of semiconductor materials, it is not suitable for forming thick, dense coatings since electrons, which are of extremely low mass, cannot provide sufficient bombardment force to densify the vapor deposit.